Internode length mutants in peas have proved extremely
valuable as research tools in plant physiology (4). For example, Gottschalk's
dwarf mutants M26 and M66 have provided the recessive mutations ls and
led,
which, when homozygous, produce plants that are deficient in GA1
(5,7). These mutants have allowed it to be shown that a clear quantitative
relationship occurs between the levels of GA1 and elongation (7).
M84B is a further dwarf mutant produced by Professor Gottschalk (Institut für
Genetik, Bonn) from cv Dippes Gelbe Viktoria (DGV) by X-irradiation (Gottschalk,
pers. comm.) and it may also be of use in physiological research. The parental
cv DGV possesses the internode length genotype leLacrycLmNaLhLsLkLkaLkbLwLv (2) and it has the longest internodes of any dwarf variety in our
collection at Hobart.

The nature of the mutation in M84B was first examined by
crossing M84B to cv DGV. The F1 possessed marginally longer
internodes than the M84B parent and was substantially shorter than the parental
cv DGV (Fig. 1). In the F2 a relatively clear segregation into 50
short and 18 long plants was obtained (Fig. 1). This is in good agreement with a
3:1 segregation (c21
= 0.08) suggesting that M84B differs from DGV by possessing a dominant mutation.
Although a clear separation across families did not occur in F3,
analysis of F3 data within families supports this conclusion since
progeny from 10 short parents segregated to give 52 short and 16 long plants (c21
testing 5:1 = 2.9) while progeny from 8 long F2 parents bred true (32
plants). If it is assumed that the genetic background is the same in DGV and
M84B the degree of dominance can be obtained directly from the F1
data and, based on the length between nodes 1 and 6, is 0.4. F3 data
support this lack of complete dominance.

Crosses of both DGV and N84B were made to the Hobart dwarf
line 61a (lelaCryLmNaLsLhLwLkLkaLkb
Lv; 6). The F2 of cross 61a x M84B yielded no clear segregation for
internode length (Fig. 2a) while the F2 of cross DGV x 61a segregated
to produce 88 dwarf (La and/or Cry) and 8 cryptodwarf (lacryc)
progeny (Fig. 2b). This latter segregation is in agreement with the expected
15:1 ratio (c21
= 0.4). The cryptodwarf plants were separated from the dwarf segregates by the
stem length between nodes 1 and 4. The dominant mutation in M84B therefore may
be at the previously established locus, Cry (1,3,6). The allele cryc
in DGV has probably been replaced in M84B by an allele comparable in effect to
Cry. However, whether this is a back-mutation to the wild-type allele at
the DNA level remains unknown.